/*
 * Copyright 2002-2012 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.skfiy.util.comparator;

import java.util.Comparator;

import org.skfiy.util.Assert;

/**
 * A Comparator that will safely compare nulls to be lower or higher than other
 * objects. Can decorate a given Comparator or work on Comparables.
 *
 * @author Keith Donald
 * @author Juergen Hoeller
 * @since 1.2.2
 * @see Comparable
 */
public class NullSafeComparator<T> implements Comparator<T> {

    /**
     * A shared default instance of this comparator, treating nulls lower than
     * non-null objects.
     */
    @SuppressWarnings("rawtypes")
    public static final NullSafeComparator NULLS_LOW = new NullSafeComparator<Object>(true);
    /**
     * A shared default instance of this comparator, treating nulls higher than
     * non-null objects.
     */
    @SuppressWarnings("rawtypes")
    public static final NullSafeComparator NULLS_HIGH = new NullSafeComparator<Object>(false);
    private final Comparator<T> nonNullComparator;
    private final boolean nullsLow;

    /**
     * Create a NullSafeComparator that sorts {@code null} based on the provided
     * flag, working on Comparables.
     * <p>When comparing two non-null objects, their Comparable implementation
     * will be used: this means that non-null elements (that this Comparator
     * will be applied to) need to implement Comparable.
     * <p>As a convenience, you can use the default shared instances:
     * {@code NullSafeComparator.NULLS_LOW} and
     * {@code NullSafeComparator.NULLS_HIGH}.
     *
     * @param nullsLow whether to treat nulls lower or higher than non-null
     * objects
     * @see Comparable
     * @see #NULLS_LOW
     * @see #NULLS_HIGH
     */
    @SuppressWarnings({"unchecked"})
    private NullSafeComparator(boolean nullsLow) {
        this.nonNullComparator = new ComparableComparator();
        this.nullsLow = nullsLow;
    }

    /**
     * Create a NullSafeComparator that sorts {@code null} based on the provided
     * flag, decorating the given Comparator.
     * <p>When comparing two non-null objects, the specified Comparator will be
     * used. The given underlying Comparator must be able to handle the elements
     * that this Comparator will be applied to.
     *
     * @param comparator the comparator to use when comparing two non-null
     * objects
     * @param nullsLow whether to treat nulls lower or higher than non-null
     * objects
     */
    public NullSafeComparator(Comparator<T> comparator, boolean nullsLow) {
        Assert.notNull(comparator, "The non-null comparator is required");
        this.nonNullComparator = comparator;
        this.nullsLow = nullsLow;
    }

    public int compare(T o1, T o2) {
        if (o1 == o2) {
            return 0;
        }
        if (o1 == null) {
            return (this.nullsLow ? -1 : 1);
        }
        if (o2 == null) {
            return (this.nullsLow ? 1 : -1);
        }
        return this.nonNullComparator.compare(o1, o2);
    }

    @Override
    @SuppressWarnings("unchecked")
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (!(obj instanceof NullSafeComparator)) {
            return false;
        }
        NullSafeComparator<T> other = (NullSafeComparator<T>) obj;
        return (this.nonNullComparator.equals(other.nonNullComparator) && this.nullsLow == other.nullsLow);
    }

    @Override
    public int hashCode() {
        return (this.nullsLow ? -1 : 1) * this.nonNullComparator.hashCode();
    }

    @Override
    public String toString() {
        return "NullSafeComparator: non-null comparator [" + this.nonNullComparator + "]; "
                + (this.nullsLow ? "nulls low" : "nulls high");
    }
}
